1. Field of the Invention
The present invention relates to liquid or gaseous propellant fueled rocket engines. More specifically, this invention relates to a throttling injector for varying propellant flow rates and consequently rocket engine thrust levels over wide ranges.
2. Description of the Prior Art
It is desirable in the employment of rocket engines to vary the thrust and therefore acceleration of the vehicle. When a rocket engine is used to repetitively fire very short duration burns, the volume of flow passages between the valves and the injector (dribble volume) is very important. Propellant remaining in these passages during the engine off-times is largely wasted and the refilling of passages causes a startup time variation in the subsequent engine firing.
A rocket engine with a throttling injector produces a controlled thrust by modulating the injector flow area. Two common methods exist for the injector flow area modulation. The first method involves modulating the injector flow area by an actuating piston which is itself actively positioned by a separate control valve. The second method involves an active upstream flow control valve in series with a passively actuated (spring loaded) injector flow control device.
However, the above-described methods of modulation have several drawbacks limiting the engine throttling ratio, the maximum thrust and the combustion efficiency. In commonly used throttling injectors the throttling ratio is typically limited to 10:1 (which is the ratio of maximum thrust to minimum thrust). This limitation s due to the rapid degradation in the combustion efficiency which occurs beyond these throttling ranges. The reduced combustion efficiency in the existing design concepts is due to the significant variation in the propellant stream momentum during throttling.
In progressively wider applications of rocket engines, there has been an attendant need for throttling ratios in the range of 20:1 or greater. Accordingly, the above methods have proved unsatisfactory. The variation in the propellant stream momentum is lessened when the injector flow area is indirectly controlled by employing an upstream throttle valve. However, heretofore an accurate stream momentum control has not been feasible without complex control devices. The maximum thrust and engine size is limited in the above common methods due to the increased spray thickness at full thrust with large engines. Large spray thickness causes combustion efficiency degradation.
In U.S. Pat. No. 3,234,731 issued to D. J. Dermody, a variable thrust device and injector is disclosed. The Dermody invention is generally comprises of the combination of a thrust chamber, wherein a plug portion thereof is axially movable to vary the throat area and wherein a variable propellant injector portion includes a poppet axially movable within a propellant passage communicating with a combustion chamber portion of the thrust chamber. The propellant passage includes a plurality of lands and grooves, the grooves being tapered to eventually blend into the lands surfaces near the outlet of the passage into the combustion chamber, and a poppet carrying spring actuated wiper rings slidable over the lands so as to accomplish propellant metering as the poppet is reciprocated.
In the Dermody device, combustion occurs when the propellants are combined just downstream of the poppet. The combustion products therefore impinge directly on, and are deflected by, the plug. This configuration must therefore be operated with an off-mixture ratio keeping the combustion gases relatively cool. The plug and poppet are actuated by independent sources.
The article entitled, "Tiny Engine Combines Muscle and Fast Response," Astronautics and Aeronautics, pages 26-30, June 1983 discloses an upstream and downstream flow control mechanism in the precombustion state of the propellants. The concept employs a decoupled approach to flow control with a cavitating venturi providing a constant mixture ratio of propellants independent of chamber pressure, injector pressure differentials or total flow. The cavitating venturi requires a high upstream pressure due to fluid friction losses. The concept utilizes a single injector pintle element which limits the possible turndown ratio. The pintle element is passively actuated (spring loaded).